Medical Research Council Centre for Reproductive Health, Queen's Medical Research Institute, University of Edinburgh, Edinburgh EH16 4TJ, United Kingdom;
Medical Research Council Human Genetics Unit, University of Edinburgh, Western General Hospital, Edinburgh EH4 2XU, United Kingdom.
Proc Natl Acad Sci U S A. 2017 Jun 13;114(24):6310-6315. doi: 10.1073/pnas.1610417114. Epub 2017 May 30.
Regulation of mRNA translation is a major control point for gene expression and is critical for life. Of central importance is the complex between cap-bound eukaryotic initiation factor 4E (eIF4E), eIF4G, and poly(A) tail-binding protein (PABP) that circularizes mRNAs, promoting translation and stability. This complex is often targeted to regulate overall translation rates, and also by mRNA-specific translational repressors. However, the mechanisms of mRNA-specific translational activation by RNA-binding proteins remain poorly understood. Here, we address this deficit, focusing on a herpes simplex virus-1 protein, ICP27. We reveal a direct interaction with PABP that is sufficient to promote PABP recruitment and necessary for ICP27-mediated activation. PABP binds several translation factors but is primarily considered to activate translation initiation as part of the PABP-eIF4G-eIF4E complex that stimulates the initial cap-binding step. Importantly, we find that ICP27-PABP forms a complex with, and requires the activity of, eIF4G. Surprisingly, ICP27-PABP-eIF4G complexes act independently of the effects of PABP-eIF4G on cap binding to promote small ribosomal subunit recruitment. Moreover, we find that a cellular mRNA-specific regulator, Deleted in Azoospermia-like (Dazl), also employs the PABP-eIF4G interaction in a similar manner. We propose a mechanism whereby diverse RNA-binding proteins directly recruit PABP, in a non-poly(A) tail-dependent manner, to stimulate the small subunit recruitment step. This strategy may be particularly relevant to biological conditions associated with hypoadenylated mRNAs (e.g., germ cells/neurons) and/or limiting cytoplasmic PABP (e.g., viral infection, cell stress). This mechanism adds significant insight into our knowledge of mRNA-specific translational activation and the function of the PABP-eIF4G complex in translation initiation.
mRNA 翻译的调节是基因表达的一个主要控制点,对生命至关重要。至关重要的是,帽结合真核起始因子 4E(eIF4E)、eIF4G 和多聚(A)尾结合蛋白(PABP)的复合物使 mRNA 环化,促进翻译和稳定性。该复合物通常被靶向以调节整体翻译速率,并且也被 mRNA 特异性翻译抑制剂靶向。然而,RNA 结合蛋白对 mRNA 特异性翻译激活的机制仍知之甚少。在这里,我们解决了这一缺陷,重点研究了单纯疱疹病毒 1 蛋白 ICP27。我们揭示了与 PABP 的直接相互作用,足以促进 PABP 的募集,并且是 ICP27 介导的激活所必需的。PABP 结合几种翻译因子,但主要被认为作为 PABP-eIF4G-eIF4E 复合物的一部分激活翻译起始,该复合物刺激初始帽结合步骤。重要的是,我们发现 ICP27-PABP 与 eIF4G 形成复合物,并需要 eIF4G 的活性。令人惊讶的是,ICP27-PABP-eIF4G 复合物独立于 PABP-eIF4G 对帽结合的影响发挥作用,以促进小核糖体亚基募集。此外,我们发现细胞 mRNA 特异性调节剂Deleted in Azoospermia-like (Dazl) 也以类似的方式利用 PABP-eIF4G 相互作用。我们提出了一种机制,即各种 RNA 结合蛋白以非多聚(A)尾依赖性方式直接募集 PABP,以刺激小亚基募集步骤。这种策略可能与低腺苷酸化 mRNA(例如生殖细胞/神经元)和/或细胞质 PABP 有限(例如病毒感染、细胞应激)相关的生物学条件特别相关。该机制为我们对 mRNA 特异性翻译激活以及 PABP-eIF4G 复合物在翻译起始中的功能的认识增添了重要的见解。